Planar fluorescent lamps are useful in many applications, including backlights for displays, such as liquid crystal displays. A common weakness in such fluorescent lamps is their limited illumination range.
Some planar fluorescent lamps utilize an electric plasma arc discharge through a mercury vapor gas to produce ultraviolet energy in a process referred to as hot cathode operation. The ultraviolet energy strikes a fluorescent material which converts the ultraviolet energy to visible light. To produce the electric plasma arc discharge, such lamps typically require a substantial minimum energy input. If the lamps are driven below the minimum energy input, the electric plasma arc discharge may not be formed, or may be highly non-uniform. Moreover, the efficiencies of such lamps can be degraded substantially at low level operation. To improve uniformity and efficiency, such lamps typically must be driven well above their minimum energy input levels so that a continuous, uniform electric plasma arc discharge can be formed. At such high energy levels, the lamp emits a substantial amount of light, typically in a range exceeding 250 foot-lamberts.
While such light intensities may be useful in relatively high light applications, in some applications such a high level of light intensity can be detrimental. For example, when such high intensity fluorescent lamps are used to provide illumination for nighttime displays in aircraft, high levels of light make it difficult for pilots to view objects outside of the cockpit or to see dimly lit instruments within the cockpit. Consequently, it is often desirable to dim the lamps to levels well below 250 foot-lamberts. Often it is desirable to dim the lamps to levels well below 10 foot-lamberts.
To improve dimmability, a filter can be added to such lamps to block out some of the light. Such filtering can reduce the maximum light intensity of the lamps, rendering them ineffective in high ambient light environments.
Another approach to producing low level illumination is cold cathode operation of fluorescent lamps. In cold cathode operation, an electric potential capacitively coupled across a set of electrodes energizes ions in the lamp chamber and causes secondary electron emission from the electrodes. The ions and electrons transfer energy to mercury vapor in the lamp causing the mercury vapor to emit ultraviolet energy. Fluorescent material in the lamp converts the ultraviolet energy to visible light. Cold cathode lamps typically produce lower light levels than hot cathode lamps, because operating cold cathode lamps at extremely high voltages can produce high voltage gradients within the lamp causing breakdown of the electrodes.